The following discussion of the background to the invention is included to explain the context of the invention. This is not to be taken as an admission that any of the material referred to was published, known or part of the common general knowledge in the art as at the priority date established by the present application.
Electrodes within a reduced pressure region of a mass spectrometer which provide electric fields for forming or containing and propagating an ion beam, or for controlling the properties of an ion beam, or for mass filtration of ions, or for affecting other aspects of an ion beam relevant to the stable operation of a mass spectrometer, usually have polished surfaces for providing an equipotential boundary for an electric field. However such electrodes are subject to deposition of non-conducting (dielectric) substances thereon. Such dielectric deposits, which generally form a film, can arise from several sources including contaminants and chemically active species in ion beams representative of the composition of analytical samples presented to the mass spectrometer for analysis. Thus an ion beam that passes through a mass spectrometer can include chemically active particles that can cause deposition of a dielectric film when they strike an electrode. The dielectric film can then cause build-up of electric charge on the surface of the electrode when charged particles contact the film. This surface charge causes unstable performance of the mass spectrometer. Sometimes a chemically reactive residual gas present in the vacuum system of a mass spectrometer can initiate the film deposition process when the gas comes into contact with the surfaces of electrodes in the vacuum system. For example residual oil vapour (hydrocarbons) from vacuum pumps can initiate the growth of dielectric films on the surfaces of electrodes. The rate of accumulation of such films can be increased greatly when the deposition process is supplemented by ion and/or electron and/or photon bombardment of the affected surfaces. Such conditions are present in many mass spectrometers and are believed to be responsible for the deposition of dielectric films that very often can be found, for example, on the ion optics and on the fringe rods of a quadrupole mass analyser in an inductively coupled plasma mass spectrometer. Residual oil vapour accompanied by ion bombardment can produce hydrocarbon-based dielectric or semi-dielectric films on these components. These dielectric films can be highly detrimental to the stability of the instrument's performance.
An object of the present invention is to provide an electrode for use in a region of a mass spectrometer in which the likelihood of deposition of dielectric substances onto the electrode is reduced.